Access schemes where user equipment transmissions need to be time aligned at the base station receiver require timing adjustments of the user equipment transmission to compensate for varying propagation delay between user equipments. This to avoid having to use unnecessary large time guard between users when having different and varying propagation delay between user equipments and base station. Basically all Time Division Multiple Access (TDMA) based access schemes such as Global System for Mobile communication (GSM), Personal Digital Cellular (PDC) and Time Division Multiple Access (TDMA) use some sort of time alignment control.
Time alignment adjustments of user equipments transmission are typically performed by using a reference signal as time reference, i.e. a base station transmitted time reference signal received in the user equipment, and order the user equipment to change the timing of the transmission in relation to the received time reference signal. The typical purpose of the time alignment adjustments is to avoid overlap between user equipments transmission at the base station receiver and make it possible to keep a fix time between reception and transmission to a certain user equipment in the base station.
Current time alignment solutions use shortened bursts, providing a large guard time at handover, avoid overlap in time between user equipment transmissions before timing adjustments have been made and also allows the base station receiver to measure the time adjustments required and also support for signaling that adjustment value to the user equipment before the user equipment starts to use normal bursts with no margin. Shortened bursts transmission and time alignment measurements are used at call setup and at handover. Shortened bursts are short enough to allow for enough time margins for the largest cells and may or may not contain user data payload. The shortened bursts used at setup may be different from the one used for handover. Normal bursts use as little time margin as possible given the frequency and accuracy of the time adjustments of the user equipment transmission.
Existing solutions requires shortened bursts transmission to be used initially by the user equipment when connecting to a new cell after handover and before the base station have adjusted the user equipment transmission in time. Shortened bursts imply shorter or longer interruption of the connection. The user equipment also depend on a good enough down link to measure on the reference signal and receive the time alignment order from the base station to stop the shortened burst transmission and start using normal bursts and normal power levels.
An alternative to use shortened bursts is described in U.S. Pat. No. 6,035,199. This document describes an apparatus and method for performing a handoff in wireless communication system. The method includes the steps of establishing communication between a mobile subscriber and a serving cell, determining whether a target cell and the serving cell are co-located, sending a normal burst handoff command to a mobile subscriber, sending a normal burst from the mobile subscriber to the target cell including a time offset, and establishing communication between the mobile subscriber and the target cell. The apparatus relates to a wireless system including a serving cell and a target cell in communication with a mobile subscriber. The mobile subscriber is sent a normal burst handoff, including an instruction to the mobile subscriber to transmit in normal bursts and to maintain current alignment, when the target cell overlaps the serving cell.
The problem with this solution is that it is limited to the case where the serving cell and target cells are co-located, i.e. using the same base station and/or the same antenna.